Ordered Network Phases in Linear Poly(isoprene-b-styrene-b-ethylene oxide) Triblock Copolymers

The equilibrium phase behavior of 43 linear poly(isoprene-b-styrene-b- ethylene oxide) (ISO) triblock copolymer melts, with molecular weights that place these materials near the order-disorder transition, is reported. Ordered phase morphologies were characterized using small-angle X-ray scattering, transmission electron microscopy, dynamic mechanical spectroscopy, and static birefringence measurements. Interpretation of these results was aided by a modeling technique that facilitates resolution of reciprocal and real-space experimental data, leading to definitive three-dimensional morphological structures. Three distinct multiply continuous network morphologies are identified across a range of compositions between 0.1 ≤ f _o ≤ 0.3, situated between two-domain and three-domain lamellae, where f _o represents the volume fraction of O blocks. Two cubic network phases, Q ²³⁰ (core-shell double gyroid, Ia3d space group symmetry) and Q ²¹⁴ (alternating gyroid, I4 ₁32 space group symmetry), flank an orthorhombic network phase, denoted O ⁷⁰ (Fddd space group symmetry), which is positioned around the isopleth composition f _s ≈ f _I. These results provide a powerful strategy for designing network phases in linear ABC triblock copolymers when χ _AB ≈ χ _BC < χ _AC, where χ represents the Flory-Huggins interaction parameter.